Auxiliary Components and Accessories
Bearings and Seals
Bearings
The bearings in gas and steam turbines provide support and positioningfortherotatingcomponents. Radialsupportisgenerallyprovidedbyjournal，or roller bearings， and axial positioning is provided by thrust bearings. Someengines， mainly aircraft jet engines， use ball or roller bearings for radialsupport， but nearly all-industrial gas turbines use journal bearings.
A long servicelife， a high degree of reliability and economic efficiency arethe chief aims when designing bearing arrangements. To reach these criteria， design engineers examine all the influencing factors:
1.
Load and speed
2.
Lubrication
3.
Temperatures
4.
Shaft arrangements
5.
Life
6.
Mounting and dismounting
7.
Noise
8.
Environmental conditions
Rolling Bearings
The aeroderivative gas turbine design， with its low supported-weightrotors-for example， the LM 5000 HP rotor weighs1，230 lbs (558 kg)-incorporates roller bearings throughout. These do not require the large lubeoil reservoirs， coolers and pumps， or the pre-and post-lube cycle associated with journal bearing designs. Roller bearings have proven to be extremely
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rugged and have demonstrated excellent life in industrial service. Mostbearings provide reliable service for over100，000 hours. In practice， it isadvisable to replace bearings when exposed during major repairs， estimatedat50，000 hours for gas generators and100，000 hours for power turbines.
There are many roller bearing types. They are differentiated according to the direction of the main radial loads (radial bearings) or axial loads (thrust bearings)， and the type of rolling elements used， balls or rollers. Figure 13-1 shows the different types of bearings. The essential difference between ball bearings and roller bearings are that ball bearings have a lower carrying capacityand higherspeeds， while the roller bearings have higher load carrying capacity and lower speeds.
The rolling elements transmit loads from one bearing ring to the other in the direction of the contact lines. The contact angle α is the angle formed by the contact lines and the radial plane of the bearing. α refers to the nominalcontactangle，i.e.， the contact angle of the load-free bearing as seen in Figure13-2. Under axialloads， the contactangle of deep groove ball bearings， angularcontact ball bearings，etc.， increases. Under a combined load it changes from one rolling element to the next. These changing contact angles are taken into account when calculating the pressure distribution within the bearing. Ball bearings and roller bearings with symmetrical rolling elements have identical contact angles at their inner rings and outer rings. In rollerbearings with asymmetrical rollers， the contact angles at the inner rings and outer rings are not identical. The equilibrium of forces in these bearings is maintained by a force component， which is directed towards the lip. The pressure cone apex is that point on the bearing axis where the contact lines ofan angular contact bearing(i.e.， an angular contact ball bearing， a taperedroller bearing， or a spherical roller thrust bearing， intersect). The contact linesare the generatrices of the pressure cone apex. In angular contact bearings， theexternal forces act， not at the bearing center， but at the pressure cone apex. 中国航空网 www.aero.cn 航空翻译 www.aviation.cn 本文链接地址：燃气涡轮工程手册 Gas Turbine Engineering Handbook 2(79)
